The overall objective of the proposed research is to determine the role of hyperthermia-induced damage to chromatin involved in DNA synthesis in the heat induced cell death and radiosensitization of cultured mammalian cells. With regard to heat-induced cell death, the extent to which hyperthermia causes alterations in the structure of replicating (nascent) chromatin will be determined using enzymatic probes of chromatin structure. The nature of heat-induced alterations of the DNA-chromosomal protein (nucleosome) structure in replicating chromatin will be determined using gel electrophoresis of the DNA fragments (nucleosomal core and linker DNA) produced by these enzymes. The relationship between these alterations and sister chromatid exchanges will be determined. The time required for recovery from these chromatin alterations will be determined and correlated with the time of onset of heat-induced cell lysis. With regard to radiosensitization, the degree to which ionizing radiation increases the heat-induced structural alterations in replicating chromatin or increases the time required for their recovery will be determined. The extent to which heat increases thymine damage of the 5',6'-dihydroxydihydrothymine (T gamma) type in replicating DNA will be assayed.